Invalid query: 1046: No database selected

OK, for anyone else that gets this:

[1] I turned on define(‘WP_DEBUG’, true);
This hightlighted a number of plugins that gave some errors
[2] Systematically renamed these plugins by appending -broken to the plugins’ directory folder name

Found the plugin that caused the white screen of death (which was a 3rd party plugin called readers-from-rss-2-blog)
[3] Set define(‘WP_DEBUG’, false);
[4] Contacted the plugin author to await a fix
[5] Smiled again with a working WordPress site ??

I’m not getting anything about the query, I’m just getting the white screen and “No Database Selected”. This happened when I tried to update WordPress, so I don’t think it is a plugin. All my plugins were up to date. I’m going to keep looking for a solution, but if anybody knows what to do on this one then please let me know.

Thanks for posting these tips Martin.. you rock!… It is the first time I experienced this kind of error and this totally helped me to trace the source of the issue…